Strabismus (a "turned eye") occurs in 2 to 4 percent of the population. In the United States, 83,000 strabismus operations are performed annually, and 20-50 percent require re-operation. At $3,000 each, the cost of 30,000 re-operations per year is $90 million. We propose to develop clinical diagnostic instrumentation that has the potential of reducing these re-operations by 20 percent saving $18 million per year, and avoiding the risk to patients of some 6,000 surgical procedures annually. We have developed an optoelectronic research instrument for precisely measuring and analyzing the passive orbital stiffness (length-tension) curves of monkeys. Since 1985, various versions of this instrument have been used successfully to run hundreds of orbital stiffness curves on monkeys in more than 50 experiments. Although very accurate, the current research equipment is too unwieldy, too slow to set up, and too bulky for routine clinical use, We propose to transform this research instrumentation into a practical, convenient, easy-to-use prototype clinical instrument to permit quantitative forced duction tests in the operating room. Development of this new clinical instrumentation will lay the technical groundwork for Phase II -- a series of clinical tests on patients undergoing eye surgery to begin building a reference data base on the relationship of orbital stiffness curves to various forms of strabismus -- thyroid ophthalmopathy, multiple re-operations, strabismus after retinal detachment repair, orbital floor fractures, etc. -- with the long term objective of providing the clinician with new diagnostic aids in the form of computerized displays of normal and abnormal ocular length-tension curves.